Gears and Gear systems are subjected to uncertainty of design parameters and loading caused by inherent conditions, measurement and manufacturing errors. Hence, the motivation of this work is to improve the deterministic design practice of gears and gear systems. A probabilistic rigid multibody dynamic gear-pair model with design and load uncertainties has been developed and analyzed. Rigid gear-pair model was developed in multibody dynamics software ADAMS and parameter uncertainty and probabilistic analysis was performed from separate probabilistic software called NESSUS. To perform the probabilistic multibody analysis, ADAMS and NESSUS were interfaced using MATLAB. The effect of parameter uncertainty on dynamic factor, DF of a gear pair was investigated. The sensitivity of DF to five uncertain loading and design parameters were also determined. This paper has demonstrated the importance of the novel PMBD modeling approach to gear design and dynamic factor analysis. The method has brought a new dimension of design approach of gears and gear systems than traditional approach that considers a certain empirically defined dynamic rating factors. In addition to revealing system reliability or under-performance through probability of failure, it also helps designers to consider certain variables critically through the sensitivity results.